The present invention relates generally to electrical safety devices and more particularly to ground fault circuit interrupters (GFCI).
Conventional electrical appliances typically receive alternating current (AC) power from a power supply, such as an electrical outlet, through a pair of conducting lines. The pair of conducting lines, often referred to as the line and neutral conductors, enable the electrical appliance, or load, to receive the current necessary to operate.
The connection of an electrical appliance to a power supply by a pair of conducting lines creates a number of potentially dangerous conditions. In particular, there exists the risk of ground fault and grounded neutral conditions in the conducting lines. A ground fault condition occurs when there is an imbalance between the currents flowing in the line and neutral conductors. A grounded neutral condition occurs when the neutral conductor is grounded at the load. A ground fault condition is extremely dangerous and can result in serious injury.
Ground fault circuit interrupters are well known in the art and are commonly used to protect against ground fault and grounded neutral conditions. In general, GFCI devices sense the presence of ground fault and grounded neutral conditions in the conducting lines, and in response thereto, open at least one of the conducting lines between the power supply and the load to eliminate the dangerous condition.
In U.S. Pat. No. 5,177,657, to M. Baer et al, there is disclosed a ground fault interrupter circuit which interrupts the flow of current to a pair of lines extending between a source of power and a load. The ground fault interrupter circuit includes a circuit breaker comprising a normally open switch located in one or both of the lines, a relay circuit for selectively closing the normally open switch, an electronic latch circuit operable in first and second bi-stable states and a fault sensing circuit for sensing the presence of a fault condition in at least one of the lines. The electronic latch circuit causes the relay circuit to close the normally open switch and maintain the normally open switch in its closed position when the electronic latch circuit is in the first bi-stable state. The electronic latch circuit also causes the relay circuit to permit the normally open switch to return to its normally open condition when the latch circuit is in its second bi-stable state. A fault sensing circuit senses the presence of a fault condition in at least one of the lines and causes the electronic latch to latch in its second state upon detection of the fault condition.
In U.S. Pat. No. 5,418,678 to T. M. McDonald, there is disclosed an improved ground fault circuit interrupter (GFCI) device which requires manual setting following initial connection to an AC power source or termination of a power source interruption. The improved GFCI device utilizes a controlled switching device which is responsive to a load power signal for allowing the relay contact sets of the GFCI device to be closed only when power is being made available at the output or load terminals. The controlled switching device preferably comprises an opto-isolator or other type of switching device which provides isolation between the GFCI input and output terminals when the relay contact sets are open. The improved GFCI device may be incorporated into portable units, such as plug-in or line cord units, for use with unprotected AC receptacles.
In U.S. Pat. No. 4,816,957 to L. F. Irwin there is disclosed an adapter unit comprising a moisture resistant housing within which is carried an improved, self testing ground line fault interrupter device. The improved device is electrically interconnected with a connector carried externally of the adapter housing so that the unit can be plugged directly into a standard duplex outlet of an existing circuit. The apparatus includes circuitry that automatically tests the operability of the device when it is plugged into a duplex outlet without the need for manual manipulation of test buttons or other overt action by the user.
In U.S. Pat. No. 4,578,732 to C. W. Draper et al there is disclosed a wall socket type ground fault circuit interrupter having a pair of sockets, a reset button and a test button that are accessible from the front of the interrupter. The interrupter has latched snap-acting contacts and a novel latching relay structure for releasably maintaining the snap-acting contacts in a circuit making position. The snap-acting contacts permit all of the components including the monitoring toroids and the power supply to be respectively located and connected at the load side of the snap-acting contacts so that all of the circuits of the interrupter are de-energized when the contacts snap to a circuit opening position. The snap-acting contact mechanism and relay are provided with structures which provide the interrupter with a trip-free mode of contact actuation and accordingly a tease-proof snap-acting contact operation.
One drawback of GFCI devices of the type described above is that the GFCI device generally includes a large solenoid to selectively open and close the switching device. Specifically, the solenoid generally requires a constant supply of line voltage (approximately 120 volts) in order to switch and sustain the solenoid in its energized state. As a consequence, the solenoid acts as a large power drain source. In addition, the constant supply of line voltage to the solenoid causes the solenoid to heat up significantly and potentially burn out.